As with any bony structure, the spine is subject to various pathologies that compromise its load bearing and support capabilities. The spine is subject to degenerative diseases, the effects of tumors and, of course, fractures and dislocations attributable to physical trauma. In the past, spinal surgeons have tackled the thorny problems associated with addressing and correcting these pathologies using a wide variety of instrumentation and a broad range of surgical techniques. In spinal surgeries, the fusion of two or more vertebral bodies is required. The use of elongated rigid plates has been helpful in the stabilization and fixation of the lower spine, most particularly the thoracic and lumbar spine while awaiting the fusion of the two vertebral bodies.
The cervical spine can be approached either anteriorly or posteriorly, depending upon the spinal disorder or pathology to be treated. Many of the well known surgical exposure and fusion techniques of the cervical spine are described in Spinal Instrumentation, edited by Drs. Howard An and Jerome Cotler. This text also describes instrumentation that has been developed in recent years for application to the cervical spine, most frequently from an anterior approach.
The anterior approach to achieving fusion of the cervical spine has become the most popular approach. During the early years of cervical spine fusion, the fusions were performed without internal instrumentation, relying instead upon external corrective measures such as prolonged recumbent traction, the use of halo devices or Minerva casts, or other external stabilization. However, with the advent of the elongated plate customized for use in the cervical spine, plating systems have become the desired internal stabilization device when performing stabilization operations.
It has been found that many plate designs allow for a uni-corticaly or bi-corticaly intrinsically stable implant. It has also been found that fixation plates can be useful in stabilizing the upper or lower cervical spine in traumatic, degenerative, tumorous or infectious processes. Moreover, these plates provide the additional benefit of allowing simultaneous neural decompression with immediate stability.
During the many years of development of cervical plating systems, particularly for the anterior approach, various needs for such a system have been recognized. For instance, the plate must provide strong mechanical fixation that can control movement of each vertebral motion segment in six degrees of freedom. The plate must also be able to withstand axial loading in continuity with each of the three columns of the spine. The plating system must be able to maintain stress levels below the endurance limits of the material, while at the same time exceeding the strength of the anatomic structures or vertebrae to which the plating system is engaged.
Further plating systems also typically require the thickness of the plate to be small to lower its prominence, particularly in the smaller spaces of the cervical spine. Additionally, the screws used to connect the plate to the vertebrae must not loosen over time or back out from the plate. This requirement, that the bone screws do not loosen over time or back out from the plate, tends to complicate implantation of known plating systems. Such bone screw retention systems generally ensure that the bone screws placed into the vertebrae through the plating system do not back out voluntarily from the plate, but often require an additional locking step to secure the screw to the plate. Current systems without the extra step have been less than optimal, overly difficult to perform, with inadequate results.
On the other hand, while the plate must satisfy certain mechanical requirements, it must also satisfy certain anatomic and surgical considerations. For example, the cervical plating system must minimize the intrusion into the patient and reduce the trauma to the surrounding soft tissue. It is known that complications associated with any spinal procedure, and most particularly within the tight confines of cervical procedures, can be very devastating, such as injury to the, spinal cord or vertebral arteries. It has also been found that optimal plating systems permit the placement of more than one screw in each of the instrumented vertebrae.
More specifically, it is known that bone screws can be supported in a spinal plate in either a rigid or semi-rigid fashion. In a rigid fashion, the bone screws are only permitted micro-motion with no significant angular movement relative to the plate. In the case of a semi-rigid fixation, the bone screw can move somewhat relative to the plate during the healing process of the spine. It has been suggested that semi-rigid fixation is preferable for the treatment of degenerative diseases of the spine. In cases where a graft is implanted to replace the diseased vertebral body or disk, the presence of a screw capable of some angulation or translation ensures continual loading of the bone graft. This continual loading avoids stress shielding of the graft, which in turn increases the rate of fusion and incorporation of the bone graft into the spine.
Similarly, rigid screw fixation is believed to be preferable in the treatment of tumors or trauma to the spine, particularly in the cervical region. It is believed that tumor and trauma conditions are better treated in this way because the rigid placement of the bone screws preserves the neuro-vascular space and provides for immediate rigid stabilization to an area typically more unstable. It can certainly be appreciated in the case of a burst fracture or large tumorous destruction of a vertebra that immediate stabilization and preservation of the vertebral alignment and spacing is essential. On the other hand, the semi-rigid fixation is preferable for degenerative diseases because this type of fixation allows for a dynamic construct. In degenerative conditions, a bone graft is universally utilized to maintain either the disc space and/or the vertebral body itself. In most cases, the graft will settle or be at least partially resorbed into the adjacent bone. A dynamic construct, such as that provided by semi-rigid bone screw fixation, will compensate for this phenomenon.
Furthermore, known plating systems often do not permit sufficient angular freedom for bone screws relative to the plate. Generally, known plating systems have defined bores through which bone screws are placed at a predefined angle. Therefore, the operating surgeon often does not have freedom to insert the bone screws into the vertebrae as to best fit the anatomy of the individual patient. While some known systems do permit bone screw angulation, they typically are not adapted to be used with an easy-to-use bone screw retaining mechanism.
It remains desirable in the art to provide a bone screw retaining system and a bone screw removal device for use with a plating system that addresses the limitations associated with known systems, including but not limited to those limitations discussed above.